Power electronic devices are used in demanding outdoor conditions such as wind and solar generators or electric vehicles (e.g. busses). Power electronics are typically placed in protective cabinet under outdoor conditions and the structure of the outdoor cabinet is not hermetically sealed. However the electronics inside the enclosure demand that temperature and humidity stays within certain limits in order to work. The rate of environmental stress depends on impurities, temperature, relative humidity and materials.
The external environment does not meet the requirements of the optimum operating conditions and those conditions are not in the same way even possible to create as in indoor building. The changing outdoor conditions meaning mainly temperature and humidity as well as outside air contaminants that contribute in degrading the overall lifetime of the device. External environment's rapid temperature and humidity changes cause worst cases in the case of electronics such in a sun rise in the morning and after rain showers or when the outside air warms-up or cools down suddenly.
When device inside outdoor cabinet is continuously switched on there appears no problems with moisture normally because internal heat losses are enough to keep the inside air warmer than outside air. Problems mainly can arise when the device is intermittent switched on (e.g. vehicle) or in a varying weather conditions. The relative humidity is the inverse of the diurnal temperature fluctuations. The diurnal relative humidity is generally at its highest in the morning at sunrise and at its lowest in the afternoon. There are also some exceptions such as in the mountainous slopes where due to valley wind daily fluctuations in humidity can go the opposite way. Relative humidity expresses in percentage of how much water vapor in the air is observed compared to the saturation humidity. As temperature rises, relative humidity decreases and respectively when temperature goes down it increases. In this case, the inside temperature of the enclosure and relative humidity will vary accordingly. In addition, variation in the temperature inside the enclosure causes the outer exterior of the enclosure's surface to breath. Moisture is absorbed inside the enclosure into the components such as printed circuit boards and a variety of plastic surfaces.
Problems caused by moisture are related to corrosion and change of electrical properties of materials or devices which can cause short circuit and damage to the appliance. The moist conditions must be controlled or the device must be conditioned before use after a stop. The problems caused by moisture after switch-off must be solved other way than continuous heating or using dehydrator because the device should not use electricity when it is switched off for economic reasons.
In a closed space almost identical absolute humidity prevails when air is mixed and temperature is stable. Relative humidity (RH) rises when temperature decreases and water molecules start to condense into a liquid to the coldest surface.
Dehumidifier takes advantage of this phenomenon. House dehumidifiers are a common household appliance which reduces the moisture in the air. There are different kinds of dehumidifiers for different kinds of applications, but all of them follow the same principle. It is based on circulating the air through the device and moisture in the flowing air condenses on the cold surface which separates the moisture from the air. This same phenomenon happens also in air conditioner when it chills the air. Dehumidifier is an expensive way to remove moisture. Dehumidifier air filter can clog, it collects condensed water in a bucket and must be (manually) removed from the enclosure. Further, dehumidifiers are equipped with a fan that draws the enclosure air over the cooling coils. As known, fans are electro-mechanical components which can breakdown easily and need service. The evaporator is made up of coils that are located on the back of the dehumidifier unit. Evaporator coils turn cold when the dehumidifier is running and the fan draws the moist air over the coils, allowing the humidity in the air to condense. If air temperature blowing over the coils is too cold, the condensation water on the coils will turn to ice, eventually compromising or ceasing operation (cp. refrigerator).
Extra heating is another common way to reduce moisture effects. The impact of heating is just based on the fact that warmed air can hold more moisture and in consequence relative humidity decreases. Outgoing air is preventing moist air to flow into the enclosure from surrounding atmosphere.
Constantly active electric device has electric losses which generates heat all the time to the surrounding enclosure. This continuous warmer temperature inside enclosure and its immediacy of surrounding air creates an anomalous area with decreased relative humidity. Decreased relative humidity as a result of surrounding air temperature rise moves the device's operating point in the Mollier diagram to the more dry side and corrosion is less. This principle happens only when the heater is turned on and heater or heaters must be always on even when the device is switched off. However, heating consumes a lot of energy and therefore the efficiency of the device decreases.
Electronic dehumidifiers with Peltier kind of heat pumps are mainly used when there is a need for small design. Peltier has poor energy efficiency (low COP value) and same kind of problems than with normal house dehumidifier. It is inapplicable where requirements prohibit e.g. extra 24V consumers.
Ionic membrane dehumidifier uses special membrane as an ionic pump to transfer humidity at a molecular level (hydrogen and oxygen, no liquid water) out of the device enclosure. Ionic membrane dehumidifiers as a new technique are expensive at a low dehumidification capacity which is bad on the long run. The good redeeming features are that it uses small amount of electricity, there are no moving parts, the process is maintenance free and water is removed in a molecular level (means no liquid water). The effect of the chemical gases transferred possibly from the environment of use to the membrane is still unknown.
Silica gel beads are widely used as a desiccant which absorbs moisture from the surrounding environment. Moisture absorption can be of physical or chemical nature. Physical absorption in materials can happen through dissolution, diffusion or at the material surface by adsorption. Adsorption is based on weak forces of attraction (van der Waals forces, electrostatic interactions). In desiccant adsorption weak forces create weak bonds which can be dissolved by heating (reversible process).
Silica gel is a porous desiccant and water vapor is moved by multilayer adsorption and capillary condensation. Time of operation depends on the humidity of environment and the amount of silica. Silica gel bead can collect only certain amount of water. Silica gel bead must be placed inside the electric enclosure for relative humidity reduction. A reversing process required for regeneration is impossible without releasing moisture back into the electric equipment enclosure.